Liquid crystal devices for practical use include TN (twisted nematic) or STN (super-twisted nematic) mode displays using nematic liquid crystals. Those utilizing ferroelectric liquid crystals have also been proposed.
These conventional devices require a polarizer and are therefore limited in brightness of the display.
It is known that use of a polymer film having dispersed therein microencapsulated liquid crystals makes it possible to produce large-sized and still inexpensive and high-contrast liquid crystal devices requiring neither a polarizer nor an alignment layer. Encapsulating materials proposed to date include gelatin, gum arabic, polyvinyl alcohol, etc. as disclosed in JP-W-58-501631 (the term "JP-W" as used herein means an "unexamined published International patent application") and as U.S. Pat. No 4,435,047 proposed. Such polymer-dispesed liquid crystal systems also include a dispersion of liquid crystals in an epoxy resin matrix (JP-W-61-502128), a film in which phase separation between liquid crystals and a polymer is fixed on exposure to light (JP-A-61-305528; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a dispersion of liquid crystals in a special ultraviolet-curing polymer (JP-A-62-2231), and a process for filming a mixture of polyester, liquid crystals, and a solvent (JP-A-63-144321).
However, the liquid crystal devices obtained by the techniques disclosed in JP-W-58-501631, JP-W-61-502128, and JP-A-61-2231 need a high driving voltage of at least 25 V and, in most cases, from 50 to 200 V for obtaining sufficient transparency. Further, the contrast ratio achieved with the liquid crystal devices disclosed in JP-A-61-305528 and JP-A-1-62615 is 10 at the highest and, in most cases, 8 or less, which is below the level required for practical use.
In order to satisfy the above-described characteristics of liquid crystal devices which are important for practical use, i.e., low power driving properties, high contrast, and multiplex driving properties, JP-A-1-198725 discloses a liquid crystal device having such a structure that a liquid crystal material forms a continuous phase in which a polymer forms a three-dimensional network.
On the other hand, it has been proposed to control transmission and absorption of light in liquid crystal color displays by using a guest-host liquid crystal material comprising a dichroic dye having a dichroic ratio of 1 or more and a liquid crystal material having positive dielectric anisotropy as described in G. H. Heilmeir, Appl. Phys. Lett., Vol. 13, p. 91 (1968). This technique has been practically applied to TN or STN mode nematic liquid crystal displays.
The dichroic dyes used in these color displays, though excellent in light and chemical stability, are limited to those having a dichroic ratio of 1 or more, the displays using them are negative displays, composed of a colorless pattern on a colored background, and are therefore poor in visual appreciation. Hence, many attempts to achieve positive displays have hitherto been made. Such attempts include a method of using a liquid crystal having negative dielectric anisotropy and an alignment layer for vertical (homeotropic) alignment of the liquid cystal molecules as disclosed in IEEE Trans. Elect. Dev., ED-26, p. 1373 (1979); a method of using a tetrazine type dichroic dye having a negative dichroic ratio (the dichroic dye was not put into practical use due to its chemical instability) as disclosed in Mol. Cryst. Liq. Cryst. Lett., Vol. 56, p. 115 (1979); and a method utilizing a special technique for obtaining parallel (homogeneous) alignment in the display portion and homeotropic alignment in the non-display portion, i.e., the background, as disclosed in Mol. Cryst. Liq. Cryst., Vol. 74, p. 227 (1981).
However, because any of the devices obtained by these methods still requires a polarizer, capability of obtaining a bright image area was limited as discussed above. In particular, when the device is applied to high-density projection displays, strong back light enough to compensate for a reduction in opening ratio is needed, leading to an increased cost. In addition, a special technique for liquid crystal alignment is required, resulting in a reduction in yield of production of displays.
In connection to the above-described light scattering type liquid crystal color displays as disclosed in JP-W-58-501631, U.S. Pat. No. 4,435,047, JP-W-61-502128, JP-A-61-305528, JP-A-62-2231, and JP-A-63-144321, there have been proposed a display system in which a non-dichroic dye is added to a polymer layer, and a dichroic dye having a dichroic ratio of 1 or more is added to an encapsulated liquid crystal, or dichroic dyes differing in color tone are added to the respective light controlling liquid crystal layers to form a double-layered structure as disclosed in SID' 86 Digest, p. 126 (1986); a display system in which a light controlling layer comprising a liquid crystal layer containing a dichroic dye having a dichroic ratio of 1 or more and a colored light-transmitting film are laminated as disclosed in SID' 90 Digest, p. 210 (1990); and a display system in which a dichroic dye having a dichroic ratio of 1 or more is added to a liquid crystal material whose dielectric anisotropy changes from positive to negative depending on the driving frequency as disclosed in SID' 90 Digest, p. 128 (1990). All the devices obtained by these techniques, however, require a high driving voltage of at least 40 V and have poor steepness in responding as demanded for multiplex driving. Moreover, there is involved such complexity that matching of refractive index between a liquid crystal material and a transparent solid substance must be optimized or that a special liquid crystal material for two-frequency driving must be used.
In addition to the above-mentioned performance requirements, such as low power driving properties, high contrast, multiplexibility, and a bright image, the conventional liquid crystal devices have the following problems.
The devices reveal hysteresis in their electro-optical characteristics, that is, a difference between a transmittance at an increasing voltage and that at a decreasing voltage. As a result, the margin of multiplexing is reduced, giving rise to a problem in making a display with grey scale level. Further, resistivity of liquid crystal devices is reduced by the influences of light, heat, etc. during production of the devices, leading to an increase in power consumption, a reduction in life, and flicker of the displayed image due to insufficient voltage holding ratio. Furthermore, in order to produce liquid crystal devices suited to conditions of use such as a driving voltage and a temperature range, it has been necessary to sufficiently examine optimum conditions of production so as to control compatibility between a liquid crystal material and a polymer, dispersibility or phase separation of a liquid crystal material in a polymer, etc. In other words, a liquid crystal material and a polymer to be combined should be properly selected so as to optimize the electro-optical properties.
Further, the above-mentioned devices using dichroic dyes having a dichroic ratio of 1 or more provide a negative display on voltage application, which comprises a colorless transparent pattern on a background which is colored by light scattering. That is, such devices cannot be applied to projection color displays. When the devices are applied to displays of direct view type, the image area is not distinguished from the background color, making it difficult to obtain a clear hue, and an extra means such as back light must therefore be added.
Furthermore, achievement of a useful positive display requires a choice of a special liquid crystal material and a special alignment technique.